1. Field of the Invention
The present invention relates generally to cutting elements for rotary drill bits for subterranean drilling, and more specifically to cutting elements providing a controlled superabrasive contact area during a predominant portion of the useful life of the cutting element, as well as bits so equipped and methods of drilling therewith.
2. State of the Art
Rotary bits are the predominant type of drill bits employed for subterranean drilling to oil, gas, geothermal and other formations. Of the types of rotary bits employed, so-called fixed cutter or "drag" bits have garnered an ever-increasing market share over the past few decades. This market share increase is attributable to a number of factors, but significant ones must be acknowledged as the wide availability and performance of superabrasive cutting elements.
Superabrasive cutting elements in their present state typically take the form of a polycrystalline diamond compact (PDC) layer or "table" formed onto a supporting substrate, typically of a cemented or sintered tungsten carbide (WC), in a press under ultra-high pressure and temperature conditions. Other superabrasive materials are known, including thermally stable PDCs, diamond films, and cubic boron nitride compacts. The present invention has utility with cutting elements employing any superabrasive material.
Several physical configurations of superabrasive tables for cutting elements are known, including square, "tombstone" shape, and triangular. However, the most common shape is circular, backed by a circular substrate of like size. These circular superabrasive tables are usually formed substantially to size in a press, but may be cut from larger, disc-shaped blanks. The other referenced shapes are generally required to be cut from a larger, disc-shaped blank, thus generating a large volume of scrap, reducing yield during fabrication and increasing fabrication costs.
As can be seen in FIGS. 1 and 2 of the drawings, state-of-the-art, disk-shaped cutting element 10 includes a circular, PDC superabrasive table 12 of substantially constant depth mounted to a disk-shaped WC substrate 14. Superabrasive table 12 includes a cutting face 16, a cutting edge 18 at the periphery of cutting face 16, and a side 20 to the rear of cutting edge 18 (taken in the direction of cutting element travel, cutting face-first). Cutting element 10 would typically be oriented on a drill bit with at least a nominal negative backrake so that cutting face 16 "leans" away from the formation being drilled. As the cutting edge 18 and side 20 of superabrasive table 12 of cutting element 10 first contact the formation under application of weight on bit (WOB) at location 22 of cutting edge 18, it can be seen that the superabrasive contact area is extremely small in both longitudinal depth or thickness as well as width, in part due to the aforementioned backrake. Thus, for a given WOB, the responsive loading per unit surface area at the side 20 of superabrasive table 12 contacting the formation being drilled is extremely high.
Due to the circular shape of the superabrasive table 12, however, as the cutting element 10 begins to wear and a so-called "wear flat" forms at one side of cutting face 16, superabrasive table 12 and the WC substrate 14 therebehind, the contact area of the superabrasive material under WOB, or so-called Normal force applied along the axis of the drill string to which the bit is secured, increases markedly in width and therefore in total area. The increasing contact area consequently requires an increase in WOB to maintain cutting element loading in terms of load per superabrasive unit surface area in contact with the formation to continue an acceptable rate of penetration (ROP). However, as WOB increases, so does wear on the superabrasive table, as well as the likelihood of spalling and fracture damage thereto. In addition, the requirement to increase WOB may undesirably affect drilling performance in terms of reducing steerability of a bit, as well as precipitate stalling of a downhole motor when the torque required to rotate under excessive WOB is exceeded, with consequential loss of tool face orientation. As can readily be visualized by looking at the relative contact area widths at location 22, location 24 (as the cutting element is about 20% in diameter worn) and location 26 (as cutting element 10 is about 40% in diameter worn and typically approaching, if not well past, the end of its useful life), the superabrasive contact area may increase by more than an order of magnitude from the time a cutting element first engages a formation until the end of its useful life, thus requiring an attendant increase in WOB to maintain ROP in a given formation.
This undesirable increase in superabrasive contact area is present in conventional PDC cutting elements bearing constant-thickness superabrasive tables of about 0.030 inch thickness. However, as cutting elements bearing tables of greater thicknesses are developed, for example 0.070 inch and 0.100 inch uniform-thickness tables, the contact area increase is exacerbated. The increase in wear flat area for such PDC cutting elements of 13 mm (0.529 inch) diameter is illustrated in FIG. 9, wherein superabrasive contact area versus percentage of cutting face diametric wear is shown respectively by lines A, B and C for cutting elements of 0.030, 0.070 and 0.100 inch superabrasive table thickness. For each of the 0.030 inch, 0.070 inch and 0.100 inch thickness tables, the contact area more than doubles between 5% and 30% diametric wear of the superabrasive table. More significantly, for the 0.070 inch and 0.100 inch thickness superabrasive tables, contact area quickly increases in absolute terms to in excess of 0.02 square inch (the maximum superabrasive contact area for a 13 mm, 0.030 inch thick table PDC cutting element), thus necessitating substantial and undesirable WOB increases extremely early in the life of the cutting element in order to maintain the load per unit surface area of superabrasive material contacting the formation. While use of a square or tombstone-shaped cutting face, would obviously provide a relatively constant superabrasive contact area, as noted above such configurations are undesirable for other reasons. Consequently, there is a need in the art for a cutting element exhibiting a circular cutting face and superabrasive table, the term "circular" as used herein including a segment of a circle a segment or which otherwise exhibits an arcuate or nonlinear cutting edge, which provides a relatively constant superabrasive contact area during a large portion of the useful life of the cutting element.